DEC Rainbow PC100-A

Warner Losh imp at
Mon Jan 18 13:04:03 CST 2016

On Mon, Jan 18, 2016 at 10:13 AM, tony duell <ard at>

> IF the PC100-A is like the PC100-B (which I have full schematics for)...
> > All LEDs on, means, "replace the system board".
> >
> > I'm not sure, he wanted to hear that ;-)
> And it's not even right!. You can have all LEDs on with a perfectly good
> system board. A fault in the power-OK circuit in the PSU could cause it!
> OK. As has been suggested the LEDs are turned on when the hardware is
> reset (they are simply connected between the outputs of TTL latches and
> +5V,
> said latches are reset to 0 by the hardware reset signal) and then
> controlled by
> the processors. So all on suggests the processors are not running. Half of
> the
> LEDs are controlled by the 8088, the other half by the Z80. The 8088 is
> reset by
> the hardware reset signal, one of its output ports controls the Z80 reset
> line.

Yes, the 8088 is expected to load Z80 program and take it out of reset. The
interrupt wired to the z80 is the software interrupt from the 8088.

> So it is likely that the 8088 (at least) is not running. I would start
> there. Check its
> reset and clock inputs, does it seem to be accessing memory, etc.

Check its address and data bus too...


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